Re: [PATCH v3 00/30] Live Update Orchestrator

[David Hildenbrand <david@redhat.com>]

On 07.08.25 03:44, Pasha Tatashin wrote:
> This series introduces the LUO, a kernel subsystem designed to
> facilitate live kernel updates with minimal downtime,
> particularly in cloud delplyoments aiming to update without fully
> disrupting running virtual machines.
> 
> This series builds upon KHO framework by adding programmatic
> control over KHO's lifecycle and leveraging KHO for persisting LUO's
> own metadata across the kexec boundary. The git branch for this series
> can be found at:
> 
> https://github.com/googleprodkernel/linux-liveupdate/tree/luo/v3
> 
> Changelog from v2:
> - Addressed comments from Mike Rapoport and Jason Gunthorpe
> - Only one user agent (LiveupdateD) can open /dev/liveupdate
> - Release all preserved resources if /dev/liveupdate closes
>    before reboot.
> - With the above changes, sessions are not needed, and should be
>    maintained by the user-agent itself, so removed support for
>    sessions.
> - Added support for changing per-FD state (i.e. some FDs can be
>    prepared or finished before the global transition.
> - All IOCTLs now follow iommufd/fwctl extendable design.
> - Replaced locks with guards
> - Added a callback for registered subsystems to be notified
>    during boot: ops->boot().
> - Removed args from callbacks, instead use container_of() to
>    carry context specific data (see luo_selftests.c for example).
> - removed patches for luolib, they are going to be introduced in
>    a separate repository.
> 
> What is Live Update?
> Live Update is a kexec based reboot process where selected kernel
> resources (memory, file descriptors, and eventually devices) are kept
> operational or their state preserved across a kernel transition. For
> certain resources, DMA and interrupt activity might continue with
> minimal interruption during the kernel reboot.
> 
> LUO provides a framework for coordinating live updates. It features:
> State Machine: Manages the live update process through states:
> NORMAL, PREPARED, FROZEN, UPDATED.
> 
> KHO Integration:
> 
> LUO programmatically drives KHO's finalization and abort sequences.
> KHO's debugfs interface is now optional configured via
> CONFIG_KEXEC_HANDOVER_DEBUG.
> 
> LUO preserves its own metadata via KHO's kho_add_subtree and
> kho_preserve_phys() mechanisms.
> 
> Subsystem Participation: A callback API liveupdate_register_subsystem()
> allows kernel subsystems (e.g., KVM, IOMMU, VFIO, PCI) to register
> handlers for LUO events (PREPARE, FREEZE, FINISH, CANCEL) and persist a
> u64 payload via the LUO FDT.
> 
> File Descriptor Preservation: Infrastructure
> liveupdate_register_filesystem, luo_register_file, luo_retrieve_file to
> allow specific types of file descriptors (e.g., memfd, vfio) to be
> preserved and restored.
> 
> Handlers for specific file types can be registered to manage their
> preservation and restoration, storing a u64 payload in the LUO FDT.
> 
> User-space Interface:
> 
> ioctl (/dev/liveupdate): The primary control interface for
> triggering LUO state transitions (prepare, freeze, finish, cancel)
> and managing the preservation/restoration of file descriptors.
> Access requires CAP_SYS_ADMIN.
> 
> sysfs (/sys/kernel/liveupdate/state): A read-only interface for
> monitoring the current LUO state. This allows userspace services to
> track progress and coordinate actions.
> 
> Selftests: Includes kernel-side hooks and userspace selftests to
> verify core LUO functionality, particularly subsystem registration and
> basic state transitions.
> 
> LUO State Machine and Events:
> 
> NORMAL:   Default operational state.
> PREPARED: Initial preparation complete after LIVEUPDATE_PREPARE
>            event. Subsystems have saved initial state.
> FROZEN:   Final "blackout window" state after LIVEUPDATE_FREEZE
>            event, just before kexec. Workloads must be suspended.
> UPDATED:  Next kernel has booted via live update. Awaiting restoration
>            and LIVEUPDATE_FINISH.
> 
> Events:
> LIVEUPDATE_PREPARE: Prepare for reboot, serialize state.
> LIVEUPDATE_FREEZE:  Final opportunity to save state before kexec.
> LIVEUPDATE_FINISH:  Post-reboot cleanup in the next kernel.
> LIVEUPDATE_CANCEL:  Abort prepare or freeze, revert changes.
> 
> v2: https://lore.kernel.org/all/20250723144649.1696299-1-pasha.tatashin@soleen.com
> v1: https://lore.kernel.org/all/20250625231838.1897085-1-pasha.tatashin@soleen.com
> RFC v2: https://lore.kernel.org/all/20250515182322.117840-1-pasha.tatashin@soleen.com
> RFC v1: https://lore.kernel.org/all/20250320024011.2995837-1-pasha.tatashin@soleen.com
> 
> Changyuan Lyu (1):
>    kho: add interfaces to unpreserve folios and physical memory ranges
> 
> Mike Rapoport (Microsoft) (1):
>    kho: drop notifiers
> 
> Pasha Tatashin (23):
>    kho: init new_physxa->phys_bits to fix lockdep
>    kho: mm: Don't allow deferred struct page with KHO
>    kho: warn if KHO is disabled due to an error
>    kho: allow to drive kho from within kernel
>    kho: make debugfs interface optional
>    kho: don't unpreserve memory during abort
>    liveupdate: kho: move to kernel/liveupdate
>    liveupdate: luo_core: luo_ioctl: Live Update Orchestrator
>    liveupdate: luo_core: integrate with KHO
>    liveupdate: luo_subsystems: add subsystem registration
>    liveupdate: luo_subsystems: implement subsystem callbacks
>    liveupdate: luo_files: add infrastructure for FDs
>    liveupdate: luo_files: implement file systems callbacks
>    liveupdate: luo_ioctl: add userpsace interface
>    liveupdate: luo_files: luo_ioctl: Unregister all FDs on device close
>    liveupdate: luo_files: luo_ioctl: Add ioctls for per-file state
>      management
>    liveupdate: luo_sysfs: add sysfs state monitoring
>    reboot: call liveupdate_reboot() before kexec
>    kho: move kho debugfs directory to liveupdate
>    liveupdate: add selftests for subsystems un/registration
>    selftests/liveupdate: add subsystem/state tests
>    docs: add luo documentation
>    MAINTAINERS: add liveupdate entry
> 
> Pratyush Yadav (5):
>    mm: shmem: use SHMEM_F_* flags instead of VM_* flags
>    mm: shmem: allow freezing inode mapping
>    mm: shmem: export some functions to internal.h
>    luo: allow preserving memfd
>    docs: add documentation for memfd preservation via LUO

It's not clear from the description why these mm shmem changes are 
buried in this patch set. It's not even described above in the patch 
description.

I suggest sending that part out separately, so Hugh actually spots this.
(is he even CC'ed?)

-- 
Cheers,

David / dhildenb